1. Field of the Invention
The invention relates in general to an apparatus and a method for repairing a failed memory cell. More particularly, the invention relates to a semiconductor and a method for repairing a failed memory cell that directly program a fuse memory cell by a tester. Therefore, after the completion of package, the failed address can still be repair by using a high voltage circuit without being restricted by the package.
2. Description of the Related Art
For the fabrication process of a semiconductor memory device, to obtain a high yield is always important. In case that there exist significant failed memory cells in a semiconductor memory device, or even that there exists only one failed memory cell, the semiconductor memory device is undeserving as a product. While fabricating a semiconductor memory device, a device with a higher integration tends to contain more failed memory cells. Or the semiconductor memory device tends to have a higher possibility to contain failed memory cells. That is, as the integration of a semiconductor device increases, the yield is reduced.
As the technique for fabricating a semiconductor memory device becomes more and more complex, the technique for packaging is correspondingly more difficult. For example, the particles or scraps produced and remained during fabrication may inevitably cause a reduction in yield. To enhance the yield, the prior technique has developed a redundancy circuit to obtain a desired yield of semiconductor memory devices. In addition to the main memory cell array for saving binary data, a redudant memory cell array is provided to replace the failed memory cells in columns and rows of the main memory cell array. Each individual redundant memory cell is connected to individual word line and bit line. Assuming that there are thousands of failed memory cells found in the main memory cell array, these redundant memory cells can replace these failed memory cells to provide a pass or non-failure memory chip.
Using the redundant memory cells or backup memory cells to replace the failed memory cells is advantageous in enhancing the yield of memory device. However, once the amounts of the designed redundant memory cells is insufficient, that is, if the number of the failed memory cells exceeds the amount of the redundant memory cells, the device cannot be repaired to be used any more.
As mentioned above, redundant memory cells can be designed around a main memory cell array to replace the failed memory cells, so as to obtain a non-defect memory chip. The connection between the main memory cell array and the redundant memory cell is typically achieved by fuse. The fuse can be blown using a laser light beam or a current. While a failed memory cell is to be repaired, the fuse is open electrically or using laser. The fuse is remained close while no repairing work is performed.
In addition, in the circuit design of a typical semiconductor device, the fuse is used for fine tuning. Especially for a dynamic random access memory (DRAM) design, the fuse is prerequisite. The fuse is often used for redundant row and column repair and is made of polysilicon or metal. A laser is then used to program the address to be repaired.
However, during the process of repair, a laser machine is used, and the wafer is to be move for fuse location. A certain fuse is then blown to proceed the repair of the address. It is thus very time and cost consuming. In addition, after the chip is fabricated, and all the pads have been bonded to the leads of the packages, that is, after being packaged, any failed memory found afterwards can never be repaired further.